fix(tibber): deduct verkoopvergoeding (sell_fee) from feed-in sell price

Tibber's API `total` already includes the buy-side inkoopvergoeding
(verified from production data: total = spot×1.21 + energy_tax 0.11085 +
inkoopvergoeding 0.0248). Under net metering Tibber pays back
`total − verkoopvergoeding` per returned kWh (NL: EUR 0.28 -> 0.2552), so the
two EUR 0.0248 fees do NOT cancel — the feed-in price sits 0.0248 below buy.

Model the verkoopvergoeding as a first-class, always-subtracted contract
field `energy.sell_fee` (default 0.0248) instead of folding it into
`sell_adjust`. New sell formula:

    sell = total − energy_tax − sell_fee − sell_adjust

`sell_adjust` now carries only the net-metering energy-tax refund
(= −energy_tax). Applied in both the billing strategy and the /prices
endpoint; recorded in the pricing snapshot. Frontend renders the field
automatically (dynamic profile form). Docs (references, m6) corrected to
drop the wrong "fees cancel" premise.

openapi/openapi.yaml

@@ -527,9 +527,9 @@ paths:
         \ (most recent first\n    within the window, then reversed to ascending order\
         \ — identical to the\n    modbus readings pattern).\n\n    Response ``points``\
         \ carries per-slot:\n    - ``buy  = total``              (Tibber all-inclusive\
-        \ price)\n    - ``sell = total − energy_tax − sell_adjust``  (from active\
-        \ version values)\n    - ``level``                     (Tibber price level,\
-        \ may be null)\n\n    ``tariff`` is null.\n\n**Manual contracts** (kind=\"\
+        \ price)\n    - ``sell = total − energy_tax − sell_fee − sell_adjust``  (from\
+        \ active version values)\n    - ``level``                     (Tibber price\
+        \ level, may be null)\n\n    ``tariff`` is null.\n\n**Manual contracts** (kind=\"\
         manual\"):\n    ``points`` is empty.  ``tariff`` carries the four effective\
         \ prices\n    derived using the billing engine formula:\n    - ``buy_dal \
         \   = energy.buy.dal    + energy_tax + ode``\n    - ``buy_normal = energy.buy.normal\